This invention relates to a process for producing 2,6-naphthalene dicarboxylic acid (hereinunder referred to as 2,6-NDA) which is useful as a raw material for high quality polyester.
2,6-NDA has been worthy of notice as a raw material for high quality polyester having excellent thermal resistance, mechanical strength, dimensional accuracy and the like. Development of a commercial process for producing 2,6-NDA has been much in demand.
Prior arts on the production of 2,6-NDA are generally of the following three types.
(1) Processes for producing 2,6-NDA which comprise oxidizing 2,6-dimethyl naphthalene in the presence of a catalyst comprising a heavy metal and a bromine compound are disclosed in U.S. Pat. No. 3,856,855 and Japanese Patent Publication (Kokai) No. 34153/1973. In these processes, it is difficult to separate the raw material, 2,6-dimethyl naphthalene from the dimethyl naphthalene mixtures, and the amount of 2,6-dimethyl naphthalene separated is not sufficient. PA1 (2) A process for producing 2,6-NDA which comprises oxidizing 2,6-diisopropyl naphthalene in the presence of a catalyst comprising Co and Mn is disclosed in Japanese Patent Publication (Kokoku) No. 89445/1985. Though 2,6-diisopropyl naphthalene can be easily synthesized, this process involves use of an excessive amount of the catalyst. So this process is not proper from an industrial point of view. PA1 (3) Processes for producing 2,6-NDA which comprise oxidizing a 2-alkyl-6-acyl naphthalene in the presence of a catalyst containing Co and Mn or Co, Mn and Br are disclosed in Japanese Patent Publication (Kokai) Nos. 61946/1987 and 67048/1987 and U.S. Pat. No. 4,764,638. However, the yield of 2,6-NDA is insufficient in these processes.
Advantageously, 2-alkyl-6-acyl naphthalenes can be easily obtained in a high selectivity by Friedel Crafts Reaction between commercially available 2-methyl naphthalene and acetyl fluoride or butyryl fluoride. Therefore, 2-alkyl-6-acyl naphthalenes have became remarkable as a raw material for 2,6-NDA.
The present inventors have produced crude 2-methyl-6-butyryl naphthalene from 2-methyl naphthalene, propylene, carbon monoxide and hydrogen fluoride, or crude 2-methyl-6-acetyle naphthalene from 2-methyl naphthalene and acetyle fluoride, and purified the products. Then, the inventors have attempted the production of 2,6-NDA from 2-methyl-6-butyryl naphthalene or 2-methyl-6-acetyle naphthalene by oxidation under conditions which were known to be obtain high yields of 2,6-NDA from 2-methyl-6-acyl naphthalene in the prior art. However, high yields cannot be obtained from 2-methyl-6-butyryl naphthalene or 2-methyl-6-acetyle naphthalene under these conditions.
The present inventors conducted research on the production of 2,6-NDA from a 2-alkyl-6-acyl naphthalene by referring to references disclosing the process 2-methyl-6-acyl naphthalene as a raw material. However, satisfactory results could not be obtained. U.S. Pat. No. 4,886,906 dated Dec. 12, 1989 assigned to the assignee of this application discloses a process for producing 2,6-NDA which comprises oxidizing a 2-alkyl-6-acyl naphthalene with molecular oxygen in the presence of a catalyst containing cobalt, manganese, bromine, and iron or copper in an acetic acid solvent to obtain 2,6-NDA in a high yield. The present inventors further conducted extensive research for improving the process of U.S. Pat. No. 4,886,906.
In the prior art, it has not been carried to control the oxygen concentration or the oxygen partial pressure in the exhaust gas in the processes for producing 2,6-NDA from 2,6-dimethyl naphthalene or 2,6-diisoprophyl naphthalene for increasing the yield of the reaction. However, it was known that the high oxygen concentration in the process is dangerous, and that the low oxygen concentration in the process is economical, and saves energy.